The present invention relates to an apparatus for monitoring the pressure of the auxiliary energy source of a slip-controlled brake system for motor vehicles and comprising at least two switches responding in the event of the pressure falling below a pressure limit value and the nominal pressure, respectively, for releasing warning signals and influencing the function of the brake slip control.
In a conventional slip-controlled brake system, locking of a wheel during a braking operation is prevented from occurring by maintaining constant pressure or decreasing the brake pressure transferred by a brake pressure generator, for example, by a master cylinder, to the wheel brakes. In the event of malfunction of the brake slip control, the brake pressure will, therefore, have to be removed or excessively reduced resulting in a dangerous "debraking" operation. Accordingly, monitoring of the brake slip control will be required to ensure that in the event of failure caused by cutting off the control, at least a normal or uncontrolled braking operation of the motor vehicle. A reliable monitoring of the brake slip control is therefore important.
A conventional monitoring device (DE-PS No. 24 11 173) for an anti-locking vehicle brake system is equipped with a pressure sensing element cutting off the control in the event of a pressure decrease in the servo circuit, namely only upon completion of a pending braking operation.
Moreover, it is known in the art to monitor the liquid volume in the supply tank and the pressure in the hydraulic system and, in the event of a defect, to shut down the brake slip control only in part by locking the pressure decrease in the static brake circuits and eliminating the dynamic inflow into the static brake circuits (DE-OS No. 32 32 051). Keeping the pressure constant for reducing the locking risk and controlling the brake slip in the dynamic circuit, under special conditions is possible, at least until completion of the initiated controlled braking operation. However, the prior art arrangements have the disadvantage that a failure of a switch, for example, of the contact operated by the pressure sensor, will not be detected. Therefore, the present invention is directed to the problem of overcoming the disadvantages of the prior art and developing a monitoring device responding in highly reliable manner to and suitably reacting on substantially all occurring defects.